The present invention is directed to a method of making a standard tool for calibrating polarimeters that analyze stress in photoelastic material, and to a set of these standard tools.
Polariscopes and polarimeters analyze stress in photoelastic materials, such as glass. Stress is viewed through a polariscope by placing a sample of the material to be analyzed in front of a polarized light source and viewing the sample though an analyzer and full wave plate. Stress in the sample appears as blue and yellow areas when the stress direction is +/−45° to the polarized light. The more intense the color, the higher the stress for samples of equal thickness.
The stress may be measured with a polarimeter by replacing the full wave plate of the polariscope with a quarter wave plate. With the analyzer set to zero degrees, stress areas appear white. The white area is viewed while rotating the analyzer and the rotation is stopped when the white area is extinguished (dark). Sample thickness affects the readings and thus the stress is recorded as the number of degrees of rotation per unit thickness.
The polarizing films in polariscopes and polarimeters degrade over time and parts of the optical systems of these devices can become misaligned, both of which compromise the accuracy of the stress analysis results. These instruments need to be checked routinely to ascertain their accuracy.
Devices exist for ascertaining the accuracy of polariscopes and polarimeters. For example, the Babinet Soleil Compensator has a variable retardation plate that cancels phase differences in birefringent materials and can be used to check the accuracy of these instruments. However, the device is expensive and delicate and the operator must be highly skilled.
Standard tools for ascertaining the accuracy of polariscopes and polarimeters that are less sophisticated than the Babinet Soleil Compensator are also known. For example, a tempered glass disk with known birefringence is available. However, retardance can vary across the surface of the disk and precise measurements of the distance from the edge of the disk are also required. Further, the disks have one stress or birefringence level and additional levels are achieved only by stacking plural disks, which can cause further errors given the tight tolerances for measurements of the distance from the disk edge.
Another problem with polariscopes and polarimeters is that training is needed to ensure accurate operation of these instruments. Samples with high birefringence exhibit chromatic aberration so that different wavelengths of light resolve at different rotations of the analyzer. An operator can see overlapping images that complicate the determination when the birefringence has been compensated by rotation of the analyzer. The operator must be taught what to see when the analyzer has been rotated the correct amount. A simple tool for facilitating this training would be particularly advantageous.
Accordingly, the prior art tools for calibrating stress analysis devices have operational and training disadvantages.